NASA > GSFC > Astrophysics Science Division > IXO
The X-ray Polarimeter (XPOL) is an imaging polarimeter, with polarization sensitivity of 1 percent for a source with a 2–10 keV flux of 1mCrab in a 105 s exposure. XPOL utilizes a fine grid Gas Pixel Detector (GPD) to image the tracks of photoelectrons produced by incident X-rays and determine the direction of the primary photoelectron, which conveys information about the polarization of the incoming radiation. XPOL also has spectrographic capabilities with a resolution of E/ΔE of ~5 at 6 keV and timing information at few ms level. The XPOL´s FOV of 2.5 arcmin × 2.5 arcmin and its spatial resolution is a good match to the mirror point spread function (PSF). See IXO Science Performance Requirements.
Data collected by the XPOL will mainly contribute to high-energy astrophysics, such as the study of Matter under Extreme Condition,Structure and Physics of Jets, and also to test theories of Quantum Gravity.
Gas proportional counters are a well-established space technology. Virtually every X-ray observatory from Uhuru (1970) through BeppoSAX (1996) utilized some form of gas counter. Gas Pixel Detectors have been under development for several years. A GPD variant— the Time Projection Chamber polarimeter—is the heart of the GEMS SMEX mission, recently selected by NASA for a 2014 launch.